A similar actuating unit is disclosed in German published patent application 34 01 402, which is the priority application for U.S. Pat. No. 4,693,083. This document discloses an actuating unit having a valve piston which actuates the control valve of the vacuum brake power booster and is coupled to the input member. The control housing accommodates the control valve, or a sleeve in operative connection with the control housing, and extends through a wall of the master brake cylinder in sealed relationship thereto. Upon actuation, these elements plunge with their ends remote from the input member into the pressure chamber of the master cylinder. In this system, on failure of the boosting force, the input member acts upon a piston having an effective cross-sectional surface which is smaller than that piston surface upon which the boosting force is acting. The arrangement of the component parts mentioned above is chosen such that the actuating travel of the control valve corresponds to the travel of the master cylinder piston so that in the event of pre-charging of the brake system (applying of brake shoes and/or supplying of a low braking pressure), a large pedal travel or actuating travel of the input member is required.